I have noticed that the length of history does affect a bit to child migration. If you have longer history then more children has been born. Now, since it's quite common that one pair of adult may have even six children, this means that there are usually more children around than adults, which means higher probability of child to arrive.
Now, I tend to use history length of 100 years when I generate a world. I hardly ever get children migrants, and I think I know the reason; The first dwarves have already managed to breed as much as they will (4-8 children). These children have usually managed to reach the adulthood and breed as well. Now, the third generation, is very often born around 20-30 years before the embark. What this means is that, while they have already reached the adulthood, they usually don't have children themselves yet! Few exception exists but those are relatively rare.
How I have noticed this? Usually the relations of each dwarf (yes, I check those more or less always) contain one generation up and/or one generation down (parents and children) but rarely grandparents or grandchildren. What this means is that the migrants are almost always from the first or second generation, and in few cases third generation. And even those who are third generation have already reached the adulthood.
So, unless you have very specific reason to have a longer history, I suggest using something like 100 years.